Asynchronous transfer mode (ATM) networks carry fixed sized cells within the network irrespective of the applications being carried over ATM. At the network edge or at the end equipment, an ATM Adaptation Layer (AAL) maps the services offered by the ATM network to the services required by the application. There are a number of industry standards and proposed standards covering various AALs. In particular, "B-ISDN ATM Adaptation Layer Type 2 Specification," draft Recommendation I.363.2, November 1996, of ITU-T (herein referred to as AAL-2) provides for efficient ATM transport of small, delay-sensitive packets in such applications as: PBX to PBX trunking over an ATM backbone; base station to switch trunking for digital cellular/personal communications service (PCS) over an ATM infrastructure; trunking between circuit switches in voice networks; and telephony gateway to Internet.
Generally speaking, AAL-2 provides the ability to efficiently fill ATM cells with packets that are smaller than the ATM cell size. Each such "small packet" conveys data between endpoints of "Logical Link Connections." (As defined in AAL-2, each such "small packet" includes an LLC identifier, which is a "called connection ID" or CID.) Each ATM connection, or virtual circuit connection (VCC), has a capacity to support a number of LLCs. Each ATM cell, while having a single virtual circuit identifier (VCI) may have packets with different CIDs.
However, many AAL-2 applications require multiple logical link connections between multiple sources and multiple destinations. For example, in the above-mentioned digital cellular/PCS application, a base station may handle multiple calls, each to a different vocoder resource (also referred to as a vocoder set or vocoder group). Many base stations have multiple point-to-point ATM connections to multiple vocoder groups. In this case, AAL-2 is used on the point-to-point ATM connections. Unfortunately, efficiency of AAL-2 degrades significantly in these situations because the multiplexing advantage of AAL-2 relies on having many LLCs on the same ATM connection.